The present invention relates to a polarity detection apparatus, and in particular relates to an apparatus lot detecting polarity of a video synchronization signal.
Typically, a video display device, e.g. a monitor, has a plurality of display modes. In general, the frequency and the polarity of the horizontal and vertical synchronization signals are used to enable a particular mode of display.
In accordance with tile conventional approach, there are two methods for detecting the polarity of tile video synchronization signals.
The first method uses a resistor-capacitor integration circuit to integrate the video synchronization signal concerned. The output of the integration is ted to a transistor switch and tile output value of the transistor switch has the representation of the polarity of the sychronization signal. However, this kind of circuit is not easily fabricated on an integrated circuit since the presence of the capacitor. Furthermore, when this method is implemented on an integrated circuit, two pins have to be reserved for the detection of the polarity of the horizontal and vertical synchronization signals. It is not a cost effective method.
The second method uses a software of a microprocessor in a monitor control circuit. At a plurality of time points which are equally spaced apart within a predetermined time interval, The microprocessor detects the voltage level of the synchronization signal at each time point. If the number of the positive voltage level is less than that of the negative voltage level, a positive polarity of the synchronization signal is determined. It the number of the negative voltage level is less than that of the positive voltage level, a negative polarity of the synchronization signal is determined. However, this method uses a part of processor time and downgrades the performance of the processor and the video system.